1. Field of the Invention
The present invention relates to a color cathode ray tube provided with a shadow mask and a method of manufacturing the same.
2. Description of the Related Art
In general, a color cathode ray tube comprises an envelope and a substantially rectangular shadow mask. The envelope includes a panel that has a phosphor screen on its inner surface. The shadow mask is opposed to the phosphor screen in the envelope. The shadow mask has an effective surface that faces the phosphor screen, and a number of apertures are formed as electron beam passage apertures in a given array in the effective surface. The shadow mask has a function to screen three electron beams that are emitted from an electron gun by means of the apertures so that the electron beams are incident upon a three-color phosphor layers that constitute the phosphor screen.
The effective surface of the shadow mask constructed in this manner is a curved surface shaped corresponding to the shape of the inner surface of the panel. It includes a porous portion that is formed having a number of apertures in a given array and a nonporous portion that is located around the porous portion and has no apertures. The shadow mask has a skirt portion that extends perpendicularly from the peripheral edge of the nonporous portion. Beads are formed on the skirt portion.
The shadow mask is manufactured by press-forming a flat mask base that is formed having specific apertures. In this case, the mask base is first placed between a knockout and a die of a pressing machine. Then, the peripheral portion of the mask base is held by a blank holder and the die, whereby the mask base is fixed. After the mask base is then projected into a specific curved form by means of a punch, the blank holder and the die are separated from each other to expose the peripheral portion of the mask base.
Then, the knockout and the punch are moved downward so that the peripheral portion of the mask base is drawn into a space between the punch and the die. By doing this, the peripheral portion is bent substantially squarely to form the skirt portion. Thereafter, all the molds are restored to their respective original positions, and the formed shadow mask is taken out. The beads that are formed on the skirt portion have a function to pull a porous portion forming portion of the mask base in the radial direction, thereby smoothing out wrinkles from the mask base. They also serve to facilitate plastic deformation and enhance the shape retention of the formed shadow mask.
Flat tubes have recently been spread as modern color cathode ray tubes. In these tubes, the outer surface of a panel is substantially flat, having a curvature radius of 100 m or more. Normally, a porous portion of a shadow mask that is formed having electron beam passage apertures is shaped corresponding to the shape of the inner surface of the panel. In a flat tube, therefore, the effective surface of the shadow mask is substantially flat, having a curvature smaller than that of the effective surface of the shadow mask of the conventional color cathode ray tube.
If the curvature of the effective surface of the shadow mask is thus small, it is hard for the shadow mask to maintain its curved mask surface resisting its own weight or external force. Thus, if the curvature of the effective surface is reduced, the retention of the curved mask surface (hereinafter referred to as curved mask surface strength) lowers. If the curved mask surface strength is low, the effective surface of the shadow mask is inevitably deformed by a very small external force during manufacture or transportation. In this case, the distance between the inner surface of the panel and the electron beam passage apertures of the shadow mask varies, so that the electron beams emitted from the electron gun fail to land on desired phosphor layers, thereby causing a color drift.
Although the lowering of the curved mask surface strength never renders the shadow mask deformed, moreover, it inevitably causes the effective surface of the mask to be easily resonated by vibration such as a voice when the mask is incorporated in a TV set. Thus, unwanted gradation is bound to appear on the picture.